Space travel isn't easy or cheap in any respect. With a return trip, you just have a mission which is: more complicated and thus more likely to go wrong (harder), way more expensive, and rather pointless. Now, some of you may be confused by the fact that I stated a return trip is pointless. That isn't entirely true, but consider this. Once an astronaut is to land on Mars, he or she would have to wait for several months before they could start the journey back home. Now, the supplies needed to keep someone alive for that amount of time masses quite a lot, so much that no known rocket could actually carry that much to Mars. Thus, you obviously land the supplies on the Martian surface before the astronaut(s) get there, but what if the supplies can be harvested/grown on the Martian surface with equipment that masses a lot less than the supplies themselves? Well, obviously you send the equipment and save a lot of money! However, if that equipment you just sent to the Martian surface is sufficient to sustain multiple human beings indefinitely (it is), why send a return rocket? Instead of having a couple months to do valuable research, one could spend more than 30 years (not to mention that Mars One will continue sending people every two years until the colony is of a sufficient size to grow in size on its own) doing wonderful research on the Martian surface, for less money! Now, would you rather: wait longer for a more expensive mission which would accomplish less, or get a mission which costs less and establishes a permanent human settlement on another planet? I prefer the latter.
